There are lots of free books online, but while there are many large collections of out of copyright works and mainly fiction, such as Project Gutenberg, copyrighted works tend to end up fragmented into small collections.
So it's great to see collections such as the techbooksforfree.com's Free Programming and Computer Science Books collection. It's not as structured as the Gutenberg Project, as they're linking to the sites of the various copyright owners, but it's a surprisingly comprehensive collection.